This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Protein translocation through the cytoplasmic membrane in E. coli is mediated by the general secretory or Sec system that comprises a channel through the membrane, SecYEG, a motor ATPase, SecA and a chaperone SecB. We are interested in the docking between SecA and SecB. Our studies based on analytical centrifugation and static light scatter in line with size exclusion chromatography indicate that the active SecA : SecB complex has the stoichiometry of two SecA protomers bound to the tetrameric SecB. In addition to its function as a chaperone, SecB couples the ATPase activity of SecA to efficient translocation. We have mutants that populate a complex with a stoichiometry of one SecA protomer to a SecB tetramer and they are defective in the coupling. Our studies with site-directed spin labeling and electron paramagnetic resonance spectroscopy have defined regions of contact between SecA and SecB. However we can not determine the orientation of SecB relative to the SecA subunits. If we could obtain the overall shape of these complexes our attempts to understand the mechanism of coupling would be greatly facilitated. The SecA motor is conserved in all bacterial species and thus its function is of wide interest.